Radio continuum size evolution of star-forming galaxies over 0.35 < z < 2.25

Abstract

We present the first systematic study of the radio continuum size evolution of star-forming galaxies (SFGs) over the redshift range $0.35<z<2.25$. We use the VLA COSMOS 3GHz map (noise $\mathrm{rms}=2.3\hat{\mathrm{I}}¼\mathrm{Jy}\mathrm{bea}{\mathrm{m}}^{-1}$, $\hat{I}{¸}_{\mathrm{beam}}=0.75\mathrm{arcsec}$) to construct a mass-complete sample of 3184 radio-selected SFGs that reside on and above the main-sequence (MS) of SFGs. We find no clear dependence between the radio size and stellar mass, $M_{\star }$, of SFGs with $10.5\lesssim \log (M_{\star }/{\mathrm{M}}_{\odot })\lesssim 11.5$. Our analysis suggests that MS galaxies are preferentially extended, while SFGs above the MS are always compact. The median effective radius of SFGs on (above) the MS of $R_{\mathrm{eff}}=1.5\pm 0.2$ ($1.0\pm 0.2$) kpc remains nearly constant with cosmic time; a parametrization of the form $R_{\mathrm{eff}}\propto (1+z{)}^{\hat{I}}\pm$ yields a shallow slope of only $\hat{I}\pm =-0.26\pm 0.08(0.12\pm 0.14)$ for SFGs on (above) the MS. The size of the stellar component of galaxies is larger than the extent of the radio continuum emission by a factor $\sim$2 (1.3) at $z=0.5(2)$, indicating star formation is enhanced at small radii. The galactic-averaged star formation rate surface density $(\hat{I}{\pounds }_{\mathrm{SFR}})$ scales with the distance to the MS, except for a fraction of MS galaxies ($\lesssim 10\%$) that harbor starburst-like $\hat{I}{\pounds }_{\mathrm{SFR}}$. These "hidden" starbursts might have experienced a compaction phase due to disk instability and/or merger-driven burst of star formation, which may or may not significantly offset a galaxy from the MS. We thus propose to jointly use $\hat{I}{\pounds }_{\mathrm{SFR}}$ and distance to the MS to better identify the galaxy population undergoing a starbursting phase.

21 pages, 19 figures, accepted for publication in A&A